The present invention relates to an electro-optic device of the kind appropriate for optical beam steering applications.
Agile optical beam steering is often required in applications ranging from laser radar ranging to free-space optical communications. In a free-space optical communication system, beam steering is required to maintain the lock between the transmitter and receiver when either or both may be moving or may be affected by vibrations. In laser radar ranging, beam steering provides the ability to track moving or maneuvering targets. These applications require the ability to quickly steer an optical beam to a new direction, that is, they require agile optical beam steering.
One way in which to achieve beam steering is to transmit an optical beam onto a device that will reflect the beam in a different, but controllable direction. The beam steerer provides control over the angle between the optical beam incident on the beam steerer and the optical beam reflected by the device. A high performance beam steering system should provide a minimum of loss between the incident beam and the reflected beam.
Optical beam steering can be implemented with electro-mechanical systems. Such systems generally consist of a mirror mounted on an electrical actuator. These systems provide relatively low losses for the strength of the reflected beam. However, such electro-mechanical systems are limited to low response frequencies up to the order of 1 KHz. The moving parts of an electro-mechanical system along with size and weight factors are considered to be major limitations of such a system.
Electro-optical devices for optical beam steering are also known. One such device is disclosed in U.S. Pat. No. 4,639,091, issued Jan. 27, 1987 to J.-P. Huignard et al. Huignard discloses an optical beam steerer comprising an electro-optical liquid crystal having an upper side upon which strip electrodes are disposed and a lower side upon which a common electrode, reflective at the optical beam wavelength, is disposed. A step voltage waveform is applied to the electrodes, which causes local variations in the index of refraction within the liquid crystal. These variations create a diffraction grating, which is used to provide steering of a beam incident on the device. One of the shortcomings of the invention disclosed by Huignard et al. is the poor response time of the liquid crystal molecules to the variation in voltage. Hence, the steering angle cannot be quickly changed.
Another optical beam shifter using a liquid crystal is disclosed in U.S. Pat. No. 5,018,835, issued May 28, 1991 to T. A. Dorschner. Dorschner also discloses a liquid crystal with electrodes on its upper side and lower side. Additional refractive means are interposed between the incident optical beam and the liquid crystal, which allows a thinner liquid crystal to be used. A thinner liquid crystal provides faster response time to voltage variations, and thus, faster beam steering. The invention disclosed by Dorschner is still limited by the response time of the liquid crystal material, which will only allow beam steering rates on the order of 1 to 10 KHz. An additional shortcoming is that the beam angle is only steered in one dimension, the dimension defined by the plane that is perpendicular to the orientation of the surface of the liquid crystal.
A faster electro-optical beam steering device is described by J. A. Trezza, et al. in xe2x80x9cLarge Format Smart Pixel Arrays and Their Applications,xe2x80x9d IEIEE 19th Aerospace Conference, Aspen, Colo., p. 299. It comprises modulator-emitter-detector (MED) GaAs based devices grouped together and connected in parallel to custom SRAM CMOS driver chips. Each device comprises a pixel in the array, and current technology allows the pixels to be closely spaced. Each pixel consists of quantum wells, and the refractive index of each pixel can be varied by changing an electric field applied across the pixel. Phase modulating the pixels within the array provides the capability of steering an optical beam incident on the array. Current technology provides the beam steering rates in two dimensions on the order of 20 MHz. However, the transmit efficiency of the device is quite low, where the intensity of reflected light is less than 10% of the intensity of the incident light.
Optical beam steering may also be provided by directing light through a device in which the angle of the light is changed. One such device is disclosed in U.S. Pat. No. 5,082,342, issued Jan. 21, 1992 to D. R. Wight, et al. Wight discloses an electro-optic waveguide device containing an array of electrically biasable waveguides of electro-optically active material. The array of waveguides creates a farfield diffraction grating. An electric field applied to the waveguides alters the refractive index within the waveguides. Changes in the refractive index change the diffraction angle resulting in shifts in the main lobe of the diffraction pattern generated by light sent through the device. Shifting the main lobe essentially results in steering the angle of an optical beam transmitted into the device. Wight also discloses an arrangement of the waveguides that allows for beam steering in two dimensions.
The invention disclosed by Wight provides substantially faster beam steering rates than the inventions earlier described. Choice of the appropriate electro-optically active material could provide steering rates in excess of 1 GHz. However, the device still has some limitations. Transmission of light through the device and the creation of a diffraction pattern may result in significant losses to light intensity and thus limit the utility of the device. In addition, manufacture of the device described by Wight is complicated. Creation of the two-dimensional beam steering device would require a micro-manipulation apparatus. Finally, the device provides no mechanism for compensating for beam dispersion for the optical beam incident on the device.
There exists a need in the art for an optical beam steering device that can provide high rate beam steering without significant loss of intensity to the steered optical beam. The need also exists for a device that can provide those capabilities and steer an optical beam in two dimensions. The need also exists for an optical beam steerer that can provide some measure of optical wavefront correction.
Accordingly, it is an object of the present invention to provide a method and apparatus for optical beam steering that can provide low loss, high rate beam steering. It is another object of the present invention to provide such beam steering in one and two dimensions. It is a further object of the present invention to provide for optical wavefront correction.
These and other objects of the present invention are accomplished by providing at least one layer of electro-optically active material within which is formed a chirped distributed Bragg reflector. The chirped distributed Bragg reflector is oriented in the electro-optically active material so that the direction of propagation of the chirped distributed Bragg reflector is parallel with light incident on the electro-optically active material. The chirped distributed Bragg reflector reflects optical beams at different wavelengths from the electro-optically active material after those beams have traveled different distances within the electro-optically active material. An electric field is applied across the electro-optically active material in a direction parallel to the direction of propagation of the chirped distributed Bragg reflector. The applied electric field causes changes in the local index of refraction within the electro-optically active material, where the local index of refraction will vary with the intensity of the electric field. Changes in the local index of refraction will vary the distance at which an optical beam at a specific wavelength will travel within the electro-optically active material. The varying distance will cause the wavefront of the incident optical beam to experience different delays such that the incident optical beam is reflected out of the electro-optically active material at a different angle than which it entered the device.
One embodiment of an optical beam steerer in accordance with the present invention uses a linearly varying electric field to provide beam steering. A voltage source connected to one end of a resistive layer located beneath at least one layer of electro-optically active material will cause an electric field to be present between that resistive layer and a conductive layer located on top of the electro-optically active material. The conductive layer is transparent to the incident light beam, so the light beam can travel into the electro-optically active material and is reflected by the chirped distributed Bragg reflector within the material. In this embodiment, the maximum angle of beam steering is dependant on the thickness of the electro-optically active material. Two dimensional steering is achieved by an electric field generated from a resistive layer on top of the electro-optically active material and a second electric field generated orthogonally to the first field by a resistive layer on the bottom of the electro-optically active material.
If the electro-optically active material layer or layers are constructed from a near resonant semiconductor structure, the magnitude of the maximum optical beam steering angle and the speed at which a beam can be steered is dependent upon the direction of the electric field. If the electric field is applied in the direction of the semiconductor junction, a greater maximum steering angle will be achieved, but at a slower rate. If the electric field is applied inversely to the direction of the semiconductor junction, a smaller maximum steering angle is obtained, but the steering angle rate is larger.
An alternative embodiment of the present invention uses a periodic step-wise varying electric field provided by multiple electrodes to provide beam steering. The step-wise variation in electric field causes a step-wise variation of the local index of refraction within the electro-optically active material that creates a staircase diffraction grating for light transmitted into the device. The periodic electric field is generated so that the periodic variation in the index of refraction results in a delay that causes a phase shift in portions of the incident light beam of up to 2xcfx80. The efficiency of the device is related to the number of steps in the electric field. Two dimensional beam steering is provided by generating a periodic step-wise varying electric field across the electro-optically active material in two orthogonal directions.